High molecular weight, water soluble polymers such as polyethylene oxide (PEO), polyacrylamide (PAM) and partially hydrolyzed polyacrylamide (HPAM) have been demonstrated to reduce drag in turbulent flows of aqueous liquids. The instant invention discloses new efficient drag reducing agents in aqueous liquids which are a novel class of acrylamide copolymers containing N-alkyl acrylamide or N,N-dialkyl acrylamide groups.
Polyacrylamide and hydrolyzed polyacrylamide are water soluble polymers that have been previously disclosed in the literature and have found application as drag reduction agents for aqueous solutions which is achieved through a combination of high molecular weight and chain expansion due to repulsion of pendant ionic groups along the polymer chain. These high molecular weight polymers, which are difficult to prepare, are sensitive to shear and are salt-sensitive, thereby limiting their application in highly saline systems. The polymers of the instant invention which are water soluble copolymers of acrylamide and N-alkyl or N,N-dialkyl acrylamide, henceforth called alkyl acrylamide, overcome these aforementioned deficiencies. In the compositions of the instant invention, the presence of a small amount of hydrophobic or water insoluble groups in a hydrophilic, or water soluble polymer chain imparts enhanced solution viscosity relative to a hydrophilic homopolymer such as polyacrylamide of comparable chain length or molecular weight to copolymers of the instant invention.
Synthesis of associating copolymers of the instant invention presents difficulties. The incompatibility of the comonomer pair prevents an effective concentration of one or the other copolymerizing species from being achieved at the locus of the polymerization of the other comonomer. These problems arise during the incorporation of a water soluble monomer into a predominantly hydrocarbon, water insoluble polymer and also in the incorporation of a water insoluble, hydrophobic monomer into a hydrophilic or water soluble polymer. The process of the instant invention overcomes these aforementioned difficulties and permits the formation of novel copolymers of acrylamide and N-alkyl acrylamide which are effective drag reduction agents for aqueous or saline solutions. This process comprises the dispersion of a solvent insoluble monomer into a microemulsion with the incompatible comonomer being dissolved in the solvent, wherein microemulsions are dispersions of droplets of oil in water or water in oil, which have a very large interfacial area per unit mass, thereby bringing these incompatible phases into close spatial proximity. The process of the instant invention comprises the steps of: preparing a surfactant solution which comprises a mixture of a surfactant such as Tween 60, an aliphatic alcohol such as n-pentanol and an alkane such as hexadecane; adding an N-alkyl acrylamide monomer such as dodecyl acrylamide to the surfactant solution; adding water to the surfactant solution with N-alkyl acrylamide monomer; adding with stirring the acrylamide monomer to the mixture of the surfactant solution and water; adding an initiator such as potassium persulfate when the temperature of the reaction solution has reached 50.degree. C. to initiate polymerization and terminatng polymerization after at least 2 hours by precipitation into acetone.
An alternative process for preparing the instant copolymers comprises the steps of forming a mixture of sodium dodecyl sulfate, acrylamide monomer and alkyl acrylamide monomer under a nitrogen atmosphere; adding deoxygenated water to the mixture to form a reaction solution; heating the reaction solution to at least 50.degree. C. ; adding a free radical initiator to the reaction solution to initiate polymerization of the acrylamide monomer and the alkyl acrylamide monomer; and polymerizing the acrylamide monomer and the alkyl acrylamide monomer to form the copolymer of acrylamide/ alkyl acrylamide.
The water soluble polymers of the instant invention which are the drag reduction agents characterized by the formula: ##STR2## wherein R.sub.1 is a C.sub.6 to C.sub.22 alkyl group, straight chained or branched, or cycloalkyl group, more preferably C.sub.6 to C.sub.20, and most preferably C.sub.6 to C.sub.18 ; R.sub.2 is the same or different alkyl group as R.sub.1 or hydrogen; x is about 90.0 to about 99.9 mole %, more preferably about 95.0 to about 99.8, and most preferably about 97 to about 99.5; and y is about 0.1 to about 10.0 mole %, more preferably about 0.2 to about 5.0; and most preferably about 0.2 to about 3.0 mole %.
U.S. Pat. No. 4,254,249 discloses a copolymer of N,N-dimethyl acrylamide and acrylamide. The polymers of the instant invention differ from the polymers of U.S. Pat. No. 4,254,249 in that the alkyl acrylamide monomer of the instant invention has only one alkyl group which must have at least six carbon atoms, whereas the polymer of U.S. Pat. No/ 4,254,249 is formed from an N,N-dimethyl acrylamide. Another important distinction is that the alkyl acrylamide monomers of the instant invention are water insoluble, whereas N,N-dimethyl acrylamide is water soluble. This distinction necessitates the specialized process of the instant invention to form the copolymers of the acrylamide and alkyl acrylamide. Copolymers of N,N-dimethyl acrylamide and N-alkyl acrylamide are described in an article entitled, "Affinity Electrophoresis in Gels containing Hydrophobic Substituents", by Jung-Len Chen and Herbert Morawetz, The Journal of Biological Chemistry, Vol. 256, No. 7, September 10, 1981, pp. 9221-9223. The authors state that "Polyacrylamide is insoluble in organic solvents and it is, therefore, difficult to prepare acrylamide copolymers with hydrophobic monomers. We employed, therefore, N,N-dimethyl acrylamide as the main monomer constituent, since its polymers are soluble in both water and organic media."
U.S. Pat. No. 4,098,987 differs from the instant invention in that it discloses the formation of copolymers of acrylamide with acrylic esters or alkyl or alkyloxymethyl acrylamides having molecular weights from 800 to 10,000 amu. The molecular weights of these polymers are substantially different from those of the instant invention which are about 100,000 to about 4,000,000. Additionally, the wt. % of the alkyl acrylamide copolymers of this reference is 20 to 60 wt. which is substantially different from that of the instant invention.